Control Method and Power Controller for Flyback Power Converter

ABSTRACT

A control method is disclosed to have a voltage sample capable of more correctly representing an output voltage of a power converter. The power converter has a primary side and a secondary side galvanically isolated from each other. A feedback voltage is received in the primary side. A main power switch is turned OFF, starting an OFF time, during which the feedback voltage is constrained in response to the voltage sample. In a sampling time within the OFF time, the feedback voltage is sampled to update the voltage sample representing the output voltage in the secondary side. A driving signal is provided in response to the voltage sample to control the main power switch.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Taiwan PatentApplication Number 111115296 filed on Apr. 21, 2022, which isincorporated by reference in its entirety.

BACKGROUND

The present disclosure relates generally to a flyback power converter,and more particularly to a control method and a power controller in useof a flyback power converter employing primary side regulation.

Flyback topology is well known in the art of power conversion because ofsimplicity in structure and galvanic isolation between primary andsecondary sides, and is commonly adapted by power suppliers with low ormiddle output power. A flyback power converter normally has a powercontroller, which controls a main power switch to make an input powersource in a primary side energize a transformer, and to release theenergy stored in the transformer to an output power source in asecondary side. The amount of energy stored or released must be wellcontrolled to regulate the output voltage of the output power source.

Conventionally, there are two different methods that a flyback powerconverter may employ to regulate the output voltage of an output powersource: PSR (primary side regulation) and SSR (secondary sideregulation). For PSR, a power controller in a primary side timelydetects a reflective voltage of a transformer where the reflectivevoltage can represent the output voltage, so as to regulate the outputvoltage. For SSR, detection circuitry in a secondary side directlydetects the output voltage, and feeds the detection result via anisolation device, such as a photo coupler, a capacitor, anothertransformer for example, back to a power controller in a primary side,which according regulates the amount of energy stored in a transformer.

In view of BOM (bill of material), PSR is preferable, because PSR doesnot need the detection circuitry that SSR required in a secondary side.Nevertheless, for PSR, the reflective voltage can represent the outputvoltage only under certain conditions. It is always a challenge for thedesigner of a power controller to accurately and correctly detect theoutput voltage in a flyback power converter using PSR.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified. These drawings are not necessarilydrawn to scale. Likewise, the relative sizes of elements illustrated bythe drawings may differ from the relative sizes depicted.

The invention can be more fully understood by the subsequent detaileddescription and examples with references made to the accompanyingdrawings, wherein:

FIG. 1 demonstrates a flyback power converter using PSR;

FIG. 2 shows a power controller;

FIG. 3 demonstrates waveforms of signals of FIGS. 1 and 2 ;

FIG. 4 shows a power controller according to embodiments of theinvention;

FIG. 5 demonstrates a voltage limiter according to embodiments of theinvention;

FIG. 6 demonstrates waveforms of signals of FIGS. 1, 4 and 5 ; and

FIG. 7 demonstrates another voltage limiter according to embodiments ofthe invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure, or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures, or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. Particular features, structures or characteristics may beincluded in an integrated circuit, an electronic circuit, acombinational logic circuit, or other suitable components that providethe described functionality. In addition, it is appreciated that thefigures provided herewith are for explanation purposes to personsordinarily skilled in the art and that the drawings are not necessarilydrawn to scale.

FIG. 1 demonstrates flyback power converter 100 using PSR, havingprimary side PRM and secondary side SEC galvanically isolated from eachother. In primary side PRM, primary winding LP of transformer TF, mainpower switch SW1, and current-sense resistor RCS are connected in seriesbetween input power line IN and input ground line GNDIN. Powercontroller 102 provides driving signal SDRV to turn ON and OFF mainpower switch SW1. In secondary side SEC, secondary winding LS oftransformer TF and rectifier diode D0 are connected in series betweenoutput power line OUT and output ground line GNDOUT. Transformer TFfurther has auxiliary winding LA, one end of which has winding voltageVAUX. A voltage divider, consisting of resistors R1 and R2 for example,is connected in parallel with auxiliary winding LA, to provide feedbackvoltage VFB to feedback node FB of power controller 102.

When driving signal SDRV turns ON main power switch SW1 to perform ashort circuit, it is ON time TON, and input voltage VIN at input powerline IN energizes primary winding LP and transformer TF as well. Whendriving signal SDRV turns OFF main power switch SW1 to perform an opencircuit, it is OFF time TOFF, and the electromagnetic energy stored intransformer TF starts releasing to induce secondary-side current ISECwhich flows from secondary winding LS through rectifier diode D0, so asto charge output capacitor COUT and build up output voltage VOUT atoutput power line OUT. Output voltage VOUT can supply power to a load(not shown in FIG. 1 ).

FIG. 2 shows power controller 102 a, capable of being used as powercontroller 102 in FIG. 1 . Power controller 102 a has sampler 162,compensation circuit 164, and pulse-width-modulator 166. Sampler 162samples feedback voltage VFB to update and hold voltage sample VSAM.Compensation circuit 164 has transconductor GM and compensationcapacitor CCOM. Transconductor GM compares voltage sample VSAM withtarget voltage VREF to charge or discharge compensation capacitor CCOM,on which compensation voltage VCOM is accordingly built. In response tocompensation voltage VCOM and current-sense signal VCS at current-sensenode CS, pulse-width-modulator 166 provides driving signal SDRV atdriving node DRV to control ON time TON and/or the switching frequencyof main power switch SW1. Compensation circuit 164 andpulse-width-modulator 166 cooperatively provide driving signal SDRV inresponse to voltage sample VSAM to control main power switch SW1. Forexample, if voltage sample VSAM is higher than target voltage VREF,implying that output voltage VOUT is currently higher than wanted,transconductor GM accordingly lowers compensation voltage VCOM,pulse-width-modulator 166 shortens ON time TON of power switch SW1, theelectromagnetic energy stored in transformer TF and released to outputcapacitor COUT lessens, so output voltage VOUT tends to decrease.

FIG. 3 demonstrates waveforms of signals of FIGS. 1 and 2 , includingdriving signal SDRV of FIG. 2 , secondary-side current ISEC of FIG. 1 ,feedback voltage VFB of FIG. 2 in an ideal case, sampling pulse TSHgenerated by sampler 162 in FIG. 2 , and feedback voltages VFB of FIG. 2that might happen in three different real cases respectively.

ON time TON is a period of time when main power switch SW1 is turned ONby driving signal SDRV, and, in the opposite OFF time TOFF is anotherperiod of time when main power switch SW1 is turned OFF, as shown inFIG. 3 . Starting with the beginning of OFF time TOFF, transformer TFreleases the electromagnetic energy stored to induce secondary-sidecurrent ISEC charging output capacitor COUT. The period of time whensecondary-side current ISEC is positive is referred to as discharge timeTDIS of transformer TF, as demonstrated in FIG. 3 . According to someembodiments of the invention, discharge time TDIS of transformer TFmight refer to the period from the beginning of OFF time TOFF to themoment when feedback voltage VFB fails across 0V. It can be seen fromwaveform 62 in FIG. 3 that during discharge time TDIS feedback voltageVFB in an ideal case is about of a constant that reflects output voltageVOUT, so sampler 162 samples feedback voltage VFB when sample pulse TSHappears during sampling time tsam, to update and provide voltage sampleVSAM in FIG. 2 , equivalently detecting output voltage VOUT.

Nevertheless, feedback voltage VFB in a real case differs from that inan ideal case, as shown in FIG. 3 . Unlike waveform 62 in FIG. 3 ,waveforms 64, 66, and 68 show that feedback voltage VFB in a real casemight not be of a constant during discharge time TDIS. Feedback voltageVFB in the beginning of discharge time TDIS in a real case mightovershoot, undershoot, or vibrate due to parasitic resistance,capacitance or inductance, and it might take a long time for feedbackvoltage VFB to stabilize and to be capable of reflecting output voltageVOUT, as demonstrated by waveforms 64, 66, and 68 in FIG. 3 . Therefore,the timing of sampling time tsam is critical. If sampling time tsamappears before feedback voltage VFB becomes stable, voltage sample VSAMdoes not represent output voltage VOUT correctly, and output voltageVOUT can not be well regulated. In some circumstances, discharge timeIDIS itself is so brief that feedback voltage VFB can not stabilizebefore the end of discharge time IDIS, and as a result voltage sampleVSAM can never be a good representative of output voltage VOUT no matterwhen sampling time tsam appears.

FIG. 4 shows power controller 102 b according to embodiments of theinvention. Power controller 102 b can be used as power controller 102 inFIG. 1 . Some parts or portions in FIG. 4 are the same or similar withcorresponding parts or portions in FIG. 2 , and are self-explanatory inview of the teaching of FIG. 2 . In comparison with power controller 102a in FIG. 2 , power controller 102 b in FIG. 4 has voltage limiter 180in addition, which connects to feedback node FB and controls feedbackvoltage VFB in response to voltage sample VSAM. Voltage limiter 180constrains feedback voltage VFB to be in a predetermined condition inassociation with voltage sample VSAM.

FIG. 5 demonstrates voltage limiter 180 a, including operationalamplifier 182 and switch 184. Configured to be a unity-gain buffer,operational amplifier 182 duplicates at its output the voltage value ofvoltage sample VSAM. Switch 184 controls the connection between theoutput of operational amplifier 182 and feedback node FB. Simplyspeaking, voltage limiter 180 a presets feedback voltage VFB to besubstantially equal to voltage sample VSAM if switch 184 shorts theoutput of operational amplifier 182 and feedback node FB. Clamp signalSCLP turns on switch 184 during clamping time TCLP, so operationalamplifier 182 forces feedback voltage VFB to have a voltage valuesubstantially equal to voltage sample VSAM.

FIG. 6 demonstrates waveforms of signals of FIGS. 1, 4 and 5 , includingdriving signal SDRV of FIG. 4 , secondary-side current ISEC of FIG. 1 ,feedback voltage VFB of FIG. 4 in an ideal case, sampling pulse TSHgenerated by sampler 162 in FIG. 4 , three feedback voltages VFB of FIG.4 that might happen in three different real cases respectively, andclamp signal SCLP defining clamping time TCLP. Some parts or portions inFIG. 6 are the same or similar with corresponding parts or portions inFIG. 3 , and are self-explanatory in view of the teaching of FIG. 3 . Incomparison with FIG. 3 , each waveform of feedback voltages VFB in FIG.6 in real cases converges and stabilizes much more quickly, and causesto have voltage sample VSAM much more reliable. From the waveforms shownin FIG. 6 , clamping time TCLP starts soon after the beginning of OFFtime TOFF, and relaxation time TFRE refers to the period betweenclamping time TCLP and sampling time tsam. Clamping time TCLP as shownin FIG. 6 is ahead of sampling time tsam. As demonstrated by waveforms84, 86 and 88, each being the waveform of feedback voltage VFB in a realcase, during clamping time TCLP voltage limiter 180 a presets feedbackvoltage VFB to be substantially equal to voltage sample VSAM. Duringrelaxation time TFRE following clamping time TCLP, voltage limiter 180 aconstrains feedback voltage VFB no more, so feedback voltage VFB isreleased to be driven by auxiliary winding LA, and the voltage dividerwith resistors R1 and R2, reflecting output voltage VOUT. Possibleovershooting, undershooting or vibration that are shown in FIG. 4 can bemostly suppressed by voltage limiter 180 a which equivalently sets theinitial condition of feedback voltage VFB during off time TOFF.Accordingly, voltage sample VSAM that sampler 162 updates and holds atsampling time tsam can represent output voltage VOUT more correctly.

According to an embodiment of the invention, each of clamping time TCLPand relaxation time TFRE is about one third of discharge time TDIS.Based on an embodiment of the invention, in a switching cycle powercontroller 102 b records length LEN of discharge time IDIS oftransformer TF, and in the next switching cycle takes a beginningportion of OFF time TOFF whose length is about one third of length LENas clamping time TCLP, during which voltage limiter 180 a constrainsfeedback voltage VFB.

Even though voltage limiter 180 a in FIG. 5 presets feedback voltage VFBduring clamping time TCLP, but this invention is not limited to. FIG. 7demonstrates voltage limiter 180 b, having comparators 186, 188, andswitches 190, 192. Simply speaking, voltage limiter 180 b substantiallylimits feedback voltage VFB within a predetermined range coveringvoltage sample VSAM. For instance, this predetermined range is betweenvoltage sample VSAM minus 0.5V and voltage sample VSAM plus 0.5V. Whenfeedback voltage VFB exceeds voltage sample VSAM plus 0.5V, comparator186 turns ON switch 190, pulling down feedback voltage VFB; and whenfeedback voltage VFB becomes below voltage sample VSAM minus 0.5V,comparator 188 turns ON switch 192, pulling up feedback voltage VFB. Inother words, when voltage limiter 180 activates, feedback voltage VFBcan change or vary only within the 1-volt range centering on voltagesample VSAM. According to one embodiment of the invention, voltagelimiter 180 activates only during OFF time TOFF before sampling timetsam, and deactivates otherwise not to affect feedback voltage VFB.Limiting feedback voltage VFB in a predetermined range, voltage limiter180 b can prevent feedback voltage VFB from being too far away fromvoltage sample VSAM because of possible overshooting, undershooting orvibration, and feedback voltage VFB can quickly converge to morereliably reflect output voltage VOUT.

While the invention has been described by way of examples and in termsof preferred embodiments, it is to be understood that the invention isnot limited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

What is claimed is:
 1. A control method in use of a power converter witha primary side and a secondary side galvanically isolated from eachother, the control method comprising: receiving a feedback voltage inthe primary side; turning OFF a main power switch to start an OFF time;and in the OFF time, constraining the feedback voltage in response to avoltage sample; in a sampling time within the OFF time, sampling thefeedback voltage to update the voltage sample, wherein the voltagesample represents an output voltage in the secondary side; and providinga driving signal in response to the voltage sample to control the mainpower switch.
 2. The control method as claimed in claim 1, wherein thestep of constraining is to preset the feedback voltage to be equal tothe voltage sample.
 3. The control method as claimed in claim 1,comprising: during a first period within the OFF time, constraining thefeedback voltage in response to the voltage sample; wherein the firstperiod is ahead of the sampling time.
 4. The control method as claimedin claim 3, comprising: recording a discharge time of a transformer ofthe power converter; wherein the length of the first period isproportional to the length of the discharge time.
 5. The control methodas claimed in claim 3, wherein the power converter comprises atransformer with a primary winding, a secondary winding, and anauxiliary winding, and the feedback voltage is provided by a voltagedivider connected to the auxiliary winding.
 6. The control method asclaimed in claim 1, wherein the step of constraining is to limit thefeedback voltage to be within a predetermined range covering the voltagesample.
 7. The control method as claimed in claim 6, wherein thepredetermined range centers on the voltage sample.
 8. A power controllerin use of a power converter with a primary side and a secondary sidegalvanically isolated from each other, the power controller comprising:a sampler sampling a feedback voltage in the primary side to update avoltage sample, wherein the voltage sample represents an output voltagein the secondary side; a compensation circuit, comparing the voltagesample with a target voltage to provide a compensation voltage; apulse-width-modulator providing a driving signal to control a main powerswitch in response to the compensation voltage; and a voltage limiterconstraining the feedback voltage in response to the voltage sample. 9.The power controller as claimed in claim 8, wherein the voltage limiterpresets feedback voltage equal to the voltage sample.
 10. The powercontroller as claimed in claim 8, wherein the voltage limiter comprises:a unity-gain buffer; and a switch connected to an output of the voltagebuffer; wherein the switch is turned ON in a first period, so theunity-gain buffer makes the feedback voltage equal to the voltagesample.
 11. The power controller as claimed in claim 8, wherein the mainpower switch is turned ON and OFF during an ON time and an OFF timerespectively, the sampler samples the feedback voltage in a samplingtime within the OFF time, the voltage limiter constrains the feedbackvoltage in a first period within the OFF time, and the first period isahead of the sampling time.
 12. The power controller as claimed in claim11, wherein the power controller records a discharge time of atransformer of the power converter; wherein the length of the firstperiod is proportional to the length of the discharge time.
 13. Thepower controller as claimed in claim 8, wherein the voltage limiterlimits the feedback voltage to be within a predetermined range coveringthe voltage sample.
 14. The power controller as claimed in claim 13,wherein the predetermined range centers on the voltage sample.
 15. Thepower controller as claimed in claim 8, wherein the power converterincludes a transformer with a primary winding, a secondary winding, andan auxiliary winding, the main power switch is connected to the primarywinding, and the feedback voltage is provided by a voltage dividerconnected to the auxiliary winding.